Many vessels in animal bodies transport fluids from one bodily location to another. Typically, fluid flows in a unidirectional manner along the length of the vessel. Varying fluid pressures over time, however, can introduce a reverse flow direction in the vessel. In some vessels, such as mammalian veins, natural valves are positioned along the length of the vessel and act as one-way check valves that open to permit the flow of fluid in the desired direction and close to prevent fluid flow in a reverse direction, i.e., retrograde flow. The valves can change from an open position in response to a variety of circumstances, including changes in the cross-sectional shape of the vessel and the fluid pressure within the vessel.
While natural valves may function for an extended time, some may lose effectiveness, which can lead to physical manifestations and pathology. For example, venous valves are susceptible to becoming insufficient due to one or more of a variety of factors. Over time, the vessel wall may stretch, affecting the ability of the valve leaflets to close. Furthermore, the leaflets may become damaged, such as by formation of thrombus and scar tissue, which may also affect the ability of the valve leaflets to close. Once valves are damaged, venous valve insufficiency may be present, and can lead to discomfort and possibly ulcers in the legs and ankles.
Current treatments for venous valve insufficiency include the use of compression stockings that are placed around the leg of a patient in an effort to force the vessel walls radially inward to restore valve function and various surgical techniques in which valves are bypassed, eliminated, or replaced with autologous sections of veins having competent valves. Drastic ablation procedures, including laser ablation, vein ligation and stripping, are also frequently employed.
Other areas of medicine have benefitted greatly over recent years from the development of minimally invasive techniques and instruments for placement of intraluminal medical devices. A wide variety of treatment devices that utilize minimally invasive technology has been developed and includes stents, stent grafts, occlusion devices, infusion catheters and the like. Minimally invasive intravascular devices have especially become popular with the introduction of coronary stents to the U.S. market in the early 1990s. Coronary and peripheral stents have been proven to provide a superior means of maintaining vessel patency, and have become widely accepted in the medical community. Furthermore, stents have been adapted for use in a variety of other treatments and medical devices.
Several researchers have pursued the development of prosthetic valves that are implantable by minimally invasive techniques over recent years. Indeed, the art now contains several examples of implantable cardiac and venous valve devices. Many of these prior art devices include an expandable support frame and an attached graft member that is fashioned into a valve that regulates fluid flow through the device and, ultimately, a body vessel. For example, a graft member can be in the form of a leaflet that is attached to a support frame and movable between first and second positions. In a first position, the valve is open and allows fluid flow to proceed through a vessel in a first direction, and in a second position the valve is closed to prevent fluid flow in a second, opposite direction. Examples of this type of prosthetic valve are described in commonly owned U.S. Pat. No. 6,508,833 to Pavcnik for a MULTIPLE-SIDED INTRALUMINAL MEDICAL DEVICE, United States Patent Application Publication No. 2001/0039450 to Pavcnik for an IMPLANTABLE VASCULAR DEVICE, and U.S. patent application Ser. No. 10/642,372, filed on Aug. 15, 2003, each of which is hereby incorporated by reference in its entirety. In other examples, a tube-like graft member that terminates in leaflets is used. The graft member is attached to one or more support frames to form a valve. The leaflets open to permit fluid flow in a first direction in response to fluid pressure on one side of the leaflets and close to prevent fluid flow in a second, opposite direction in response to fluid pressure on opposite sides of the leaflets. An example of this configuration is provided in U.S. Pat. No. 6,494,909 to Greenhalgh for AN ENDOVASCULAR VALVE, which is hereby incorporated by reference in its entirety.
Despite these and other examples, a need remains for improved implantable valve devices, methods of making such devices, and methods of treating various venous-related conditions, disorders and/or diseases.